The invention relates to a pressure control valve unit of the type such as is shown in the German Offenlegungsschrift No. 2,625,502. In this known construction, the relay valve monitors only an outside-air connection. It is also known to embody anti-locking protection pressure control valves as complete relay valves, as is shown in the German Auslegeschrift No. 1,630,544. The main advantages in doing so, besides being able to omit the special relay valves or rapid-release valves as well which are often required are as follows:
During normal braking (without anti-locking protection control):
rapid aeration and ventilation in long brake lines; PA1 rapid-release effect when releasing the brake; PA1 movement of the switching elements during each braking occurence; and PA1 load-dependent control devices with small flow-through cross sections when attached to the control circuit. PA1 a pressure gradient substantially independent of brake cylinder volume, because of the unitary effect; brake cylinders of different size as well as cylinder volumes which become larger with surface wear have no effect on the regulatory effectiveness; PA1 the inclusion and regulation of several brake cylinders is possible simultaneously, such as in the case of axle regulation.
During regulated braking (when there is anti-locking protection control):
However, the relatively large structural volume of these valves has a disadvantageous effect on the regulatory function. The long switching times caused thereby result in insufficient pressure modulation values, which do not permit sufficient regulator performance for individual wheel regulation.
Also with respect to cost, relay pressure control valves for individual wheel regulation are too expensive, since, of course, for this type of regulation, a relay pressure control valve is required for each wheel.
Relay pressure control valves therefore are most often utilized as axle regulation valves, that is, for a type of regulation in which several brake cylinders are simultaneously regulated so that they have a common brake pressure level.
In contrast, rapid-switching valves of simple design (without a relay function and without a proportional characteristic), of small structural volume, are inserted for individual wheel regulation; however, because of their functional characteristic, they do not have the advantages, which are in themselves desirable for individual wheel regulation as well, of the relay pressure control valves.